1. Field of the Invention
This invention is related to semiconductor lasers and, more particularly, to a method and apparatus for providing composite second order (CSO) and composite triple beat (CTB), or third order, distortion correction for semiconductor lasers.
2. Background
Directly modulating the analog intensity of a distributed feedback (DFB) laser is widely used to transmit analog signals, such as sound or video signals and data, on optical fibers over a long distance. Such an amplitude modulation signal typically suffers from nonlinearity of the optical source. DFB lasers are limited primarily by CSO second order distortion.
Reducing the distortion of an optical laser transmitter or other electric devices has been studied for some time. It has been increasingly necessary to extend the operation of optical laser devices into high power and larger optical modulation index (OMI) depth. Pushing a laser to an optical level output higher than its rated level is favorable, since an upgrade in optical power level is essentially achieved using lower grade, low cost lasers. Typically, the carrier to noise ratio of a laser transmitter is limited by the non-linearity of the DFB laser diode. This non-linearity results in optical modulation depth limitations and primarily CSO distortions that are impressed upon the signal transmitted. Operating the laser at or above its peak optical power rating also introduces CTB distortion. Improvements to the optical output power, the system dynamic range and the carrier-to-noise ratio can be achieved by reducing the nonlinear CSO and CTB distortions produced by the laser transmitter. Accordingly, minimizing these distortions is paramount to efficient laser operation.
Three basic ways of improving laser transmitter distortion performance include: 1) feed-forward technique; 2) multipath predistortion technique; and 3) inline predistortion technique.
The first method is the feed forward technique. Using this technique, the input signal of the laser transmitter is sampled and compared to the laser output signal to determine the difference between the signals. From this difference, the distortion component is extracted. This distortion component is then amplified by an auxiliary amplification circuit, converted back to an optical signal by another optical source, and combined with the optical output of the laser circuit such that the two distortion components cancel each other. Although this improves the distortion characteristics of the laser, the power consumed by the auxiliary amplification circuit is undesirable. This circuitry is also complex and costly.
The second method is the multipath distortion technique, in which the input source signal is split between two or more separate distortion producing paths connected in parallel. This technique requires complex system components and adjustment, thus increasing the cost and reducing the system reliability.
The third method is the inline predistortion technique, in which the input RF signal is passed in series through a distortion-producing path before the RF signal is input to the DFB laser. In this technique, nonlinear devices generate a predistortion signal that is equal in amplitude but opposite in phase to the distortion component generated by the laser. Canceling the distortion produced by the laser improves the operating characteristics of the laser. However, prior art predistortion circuits designed for correcting CSO distortion actually produce CTB distortion. Even with this limitation, inline predistortion is the simplest technique for laser distortion correction and is the favored method addressed hereinafter.
U.S. Pat. No. 5,119,392 (Childs) discloses an inline CSO predistortion circuit for use with a laser diode. The predistortion circuit includes a field effect transistor (FET) biased for square law operation that generates a mostly CSO predistortion. Due to field and doping dependent variations in carrier mobility of a FET, the actual distortion may deviate from pure CSO distortion toward CTB or odd order distortion. Since there are difficulties in achieving the ideal CSO distortion and a very good RF frequency response across wide frequency bands, such as is required for CATV applications, by using single stage FET amplifiers, the performance of this predistortion circuit is limited.
It is advantageous for a predistortion circuit to correct for both CSO and CTB distortion over a broad frequency range. However, existing prior art solutions require the use of numerous complex distortion circuits, each circuit correcting a limited portion of the broad frequency range to be transmitted by laser. For example, U.S. Pat. No. 5,523,716 (Grebliunas) discloses an in-line CTB predistortion circuit for satellite applications. Because of the different frequency range, bandwidths and power ranges, this design is inappropriate for and not transferable to CATV applications. Satellite applications operate at a much higher frequency range and over a limited frequency band. CATV applications operate over several octaves, which is much greater than satellite applications. Also, since the power in a satellite application is much greater than for a CATV application, the diodes used in a satellite application are biased at zero (0) volts. In contrast, for CATV applications, the diodes must be forward biased because of their lower RF signal power levels.
U.S. Pat. No. 6,204,718 (Pidgeon) discloses a combination of two different and separate predistortion circuits that must be combined to provide CSO distortion correction across a wide frequency range.
U.S. Pat. No. 5,600,472 (Uesaka) discloses an in-line CSO distortion circuit, as shown in FIG. 1. The effectiveness of the prior art circuit shown in FIG. 1 is limited by at least two factors. First, the RF attenuator is AC coupled to the nonlinear diode through at least one DC blocking capacitor, which in this example are capacitors C11, C12 and C13. Second, the high resistance values of the DC biasing circuit (compared to the diode difference resistance), which produces the bias voltage for diode D11, are sufficiently high that they prevent the blocking capacitors from discharging and adversely affecting the diode nonlinear correction current. U.S. Pat. No. 5,798,854 (Blauvelt et al.) discloses a CSO predistortion circuit similarly limited by resistors R22, R23 and blocking capacitor C21, as shown in FIG. 2.
Theoretically, during operation of an inline predistortion circuit, the RF signal current flows through an attenuator before flowing to the laser and the attenuator samples the RF current that the laser receives. The current sample creates a voltage across the attenuator. Nonlinear current produced by a Schottky diode connected in parallel with the attenuator provides CSO correction.
However, in the prior art, the DC blocking capacitors affect the voltage across the diode. The charge stored on the capacitors creates an average voltage, rather than an instantaneous voltage relative to the RF signal. An average voltage then results in an average correction current out of the diode. Therefore, an average inverse compensation current is used to predistort the RF signal input to the laser rather than an instantaneous current.
When such prior art circuits experience linear current from the RF input signal, the DC blocking capacitors block only the unwanted DC components. However, when the prior art circuits experience nonlinear current, the DC blocking capacitors in the inverse compensation circuit charge when the diode is forward biased (on), and discharge through high DC bias resistance when the diode is reverse biases (off). Because the resistance through which the capacitors must discharge is sufficiently large, an electrical charge accumulates and is maintained on the capacitor. The compensation circuit then provides an average compensation current rather than an instantaneous compensation current, greatly reducing the accuracy of the predistortion circuit.
The charge accumulated and maintained on the DC blocking capacitors of the prior art create an adverse affect on the predistortion circuit that is actually three-fold. First, the voltage drop across the diode is reduced, which reduces the RF drive efficiency of the diode by reducing the inverse compensation current it produces. Second, the charging and discharging of the DC blocking capacitor causes a timing offset in the inverse compensation current. The variation of the charge on the capacitor depends on the variation of the RF input signal as it goes through its positive and negative cycles charging and discharging the capacitor to the extent allowed by the time constant of the circuit. The resulting voltage change across the diode is no longer instantaneously proportional to the nonlinearity of the laser. Third, because of the average voltage stored in the DC blocking capacitor, the positive RF drive voltage across the diode is significantly less than the positive RF voltage across the RF attenuator. To apply a sufficient voltage across the diode to turn it on during the positive RF signal cycle, the resistance value of the attenuator is increased proportionally to perform voltage division between the diode and the DC coupling capacitor. To compensate for the increased resistance, the power of the RF input signal is also proportionally increased, which increases third order distortion in the signal. The RF waveform operating on the diode will be different from the RF waveform operating on the attenuator, which increases third order distortion in the laser output signal.
Based on the above, the prior art predistortion solutions clearly lack appreciation of the controlling factors for providing the most efficient and effective predistortion control.
Hence, a need exists for a predistortion circuit with improved nonlinear current levels to reduce or eliminate both the CSO and CTB distortion produced by a DFB laser diode across a broad frequency range. The present invention solves the problems of the prior art and satisfies these needs in a simple single circuit.